Included in the general class of amplifiers are fully differential operational amplifiers having differential inputs and differential outputs. Fully differential operational amplifiers have the advantageous properties of good power supply rejection and good dynamic range.
However, fully differential operational amplifiers as shown in the prior art have a significant drawback, in that they cannot be configured as unity gain circuits (that is, as fully differential unity gain operational amplifiers). Moreover, configurations of prior art dual single-ended operational amplifiers aimed at achieving unity gain typically incur a major penalty in the form of poor noise rejection and hence poor dynamic range as compared with fully differential architecture, where common mode noise cancels itself. Finally, configurations of prior art single-ended operational amplifiers aimed at achieving fully differential operation and unity gain suffer further disadvantages by requiring a large chip area and a great deal of power due to the necessary duplication of components.
Another possible alternative to fully differential architecture is passive components, but here too the prior art suffers drawbacks. There is the inability to integrate some passive components (e.g. coils), which necessitates external componentry and hence low frequency operation. Also, there are limitations on the types and performance of even those filters that can be implemented in integrated passive components.